Belt conveyors are used in a variety of situations to move material from a storage hopper at an infeed end to an outfeed end at the desired destination for the material. These belt conveyors can vary considerably in length; it is not unusual to find such conveyors covering distances from 20 to 100 feet and even longer. Such conveyors are useful for a variety of purposes. In the construction industry, for instance, they can be used to transport fill material such as clay, crushed rock, soils, or combinations thereof, for use in landfill construction sites, e.g. dams, parking lots and log yards, among others. In such situations, it is important to accurately know the weight of the material which is being moved by the conveyor, and hence, such systems typically include a scale mounted along the conveyor for weighing the material; such systems are hence known as conveyor belt scale systems.
Conveyor belt scale systems can have a variety of configurations. In one example, the conveyor system comprises an elongated supporting frame which includes a plurality of lateral roller assemblies at spaced intervals, every three feet or so, along the length of the frame for support of an endless belt. The roller assemblies can be arranged in a variety of ways. In one example, a single lateral roller assembly includes two side rollers which angle upwardly and an intermediate level roller, thereby giving the belt a trough-like appearance. The upper run of the belt moves along the top of the successive spaced roller assemblies, the belt forming a trough in accordance with the configuration of the roller assemblies. The lower or return run of the belt typically is located a short distance beneath the upper run.
In such belt scale systems, accuracy is important; a typical accuracy goal is approximately .+-.1 or .+-.2 percent. As part of the accuracy requirement, it is important that the readout display portion of the scale maintain a consistent zero. A consistent zero requires that the belt be properly tensioned. In conventional systems, the desired tension is maintained through what are known as gravity take-up systems, which comprise a series of weights of various amounts which are hung from the return run portion of the belt.
Gravity take-up systems, while effective in some situations, have several disadvantages. First, the weights are cumbersome, difficult to work with and bulky. Also, because the weights are provided in discrete amounts, the proper combination of weights to produce a particular tension may not be available. Further, it is necessary at regular intervals to stop the conveyor and re-zero the belt by adding or subtracting weights, due to physical changes in the belt itself, typically following initial operation. This is expensive and time-consuming and hence is undesirable. Lastly, and perhaps most importantly, conventional gravity take-up systems are not operable with a small or portable belt conveyor system, since there is typically not enough room under the conveyor system for the hanging weights. This is especially true for those systems which must operate within a confined space.
Hence, it would be desirable to have a system for tensioning a conveyor belt which is susceptible to the above-described disadvantages.